Summary/abstract The proteins encoded by the human genome may participate in as many as 200,000 pairwise protein interactions, and ?nearly every major process in a cell is carried out by assemblies of 10 or more interlocking protein molecules? (Bruce Alberts, Cell 1998). However, there are no commercially available expression systems for regulated expression of multi-subunit protein complexes. There is an acute need for a mammalian expression system that can produce multi-protein complexes, is tightly regulated, stable in prolonged culture, and non-integrated. No reported expression system comes near to meeting all these needs. Lucigen will develop and commercialize a stable episomal expression system with unbiased cloning ability, tunable light-inducible promoters, improved fluorescent tags, and the ability to express equivalent amounts of multiple proteins. This system will facilitate direct experimental validation of the myriad protein complexes predicted by computational models, as well as those that are already known (e.g, ion channels, multi-subunit enzymes, biochemical pathways).